1. Field of the Invention
The invention relates to a method of processing solids which have separated out in finely divided form in a liquid and which are to be processed to a dry, pourable powder with low dust content.
2. Discussion of the Background
Among the solids subject to the present method are, e.g., natural or synthetic rubbers which have precipitated out of aqueous suspensions or solutions, and which solids may also contain fillers. Manufacture of such pulverulent rubbers is described, for example, in Ger. OS No. 28 22 148, OS No. 36 06 742, and OS No. 36 06 743. Fillers which may be used include carbon black or active silicic acid, or the usual inactive fillers used in rubber compounding, as well as other components of the finished rubber product.
It is known to separate finely divided solids from suspensions by sedimentation. In the case of fine particles and small differences between the density of the solid and that of the liquid, impracticably long sedimentation times result. Accordingly, centrifuges are employed to reduce the sedimentation time. While centrifuges very effectively separate out the solid, particle damage or detrimental particle alteration is unavoidable, resulting from the mechanical transport processes (1978 Aufbereitungs-Technik, 19, 10:481-483). Breakage of particles and wear of their surfaces is observed. The result is a powder with low pourability and high dust content. Relatively soft components within the particles are liberated or exposed, whereby clumps can develop.
In the subsequent thermal drying of the powder which has been separated out, one strives for intimate contact between the powder being dried and the drying gas or the apparatus walls which may be heated. If a rapidly moving stirring device or a drying gas with high flow speed is employed for this agitation purpose, additional damage to the particles, comparable to that suffered in the separation, is unavoidable.
Accordingly, there exists a need for a process whereby a finely divided solid which has been separated out in a liquid can be converted to a pourable powder with negligible dust content, maximally narrow particle size distribution, and no damage to the particles.